WATER, carbon dioxide and sulphur are important in the evolution of magmas(1,2) and the physics of volcanic eruptions(3,4). These volatile constituents occur in magmas as dissolved species in silicate melt, but can also form bubbles of exsolved gas if the magma is gas-saturated(5). Quantifying the total (dissolved plus exsolved) preeruptive concentrations of magmatic volatiles is essential for understanding a wide range of magmatic processes. We present a method for quantifying both the amount and distribution of preeruptive exsolved gas in a crystallizing silicic magma body. Application to the 0.76-Myr-old(6) Bishop rhyolitic tuff in eastern California reveals a pre-eruptive gradient in exsolved gas, with gas contents varying from less than 2 wt% in the deeper regions of the magma body to nearly 6 wt% near the top. This gradient would have promoted stable stratification of the magma body because exsolved gas lowers bulk magma density. More generally, exsolved gas in silicic magmas could contribute to the formation of many hydrothermal ore deposits and to the fluxes of volatile species from volcanic systems.